CSS Text Decoration Module Level 3

1. Introduction

This subsection is non-normative.

This module covers text decoration, i.e. decorating the glyphs of the text once typeset according to font and typographic rules. (See [CSS-TEXT-3] and [CSS-FONTS-3].) Such features are traditionally used not only for purely decorative purposes, but also in some cases to show emphasis, for honorifics, and to indicate editorial changes such as insertions, deletions, and misspellings.

CSS Levels 1 and 2 only defined very basic line decorations (underlines, overlines, and strike-throughs) appropriate to Western typographical traditions. Level 3 of this module adds the ability to change the color, style, position, and continuity of these decorations, and also introduces emphasis marks (traditionally used in East Asian typography), and shadows (which were proposed then deferred from Level 2).

1.1. Module Interactions

This module replaces and extends the text-decorating features defined in [CSS2] chapter 16.

1.2. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

1.3. Terminology

The terms character, letter, and content language as used in this specification are defined in [CSS-TEXT-3]. Other terminology and concepts used in this specification are defined in [CSS2] and [CSS-WRITING-MODES-4].

2. Line Decoration: Underline, Overline, and Strike-Through

The following properties describe line decorations that are added to the content of an element. When specified on or propagated to an inline box, that box becomes a decorating box for that decoration, applying the decoration to all its fragments. The decoration is then further propagated to any in-flow block-level boxes that split the inline (see CSS2.1 section 9.2.1.1). When specified on or propagated to a block container that establishes an inline formatting context, the decorations are propagated to an anonymous inline box that wraps all the in-flow inline-level children of the block container. When specified on or propagated to a ruby container, the decorations are propagated only to the ruby base. For all other box types, the decorations are propagated to all in-flow children.

Note that text decorations are not propagated to any out-of-flow descendants, nor to the contents of atomic inline-level descendants such as inline blocks and inline tables. They are also not propagated to inline children of inline boxes, although the decoration is applied to such boxes.

Underlines, overlines, and line-throughs are drawn only for non-replaced inline boxes, and are drawn across all text (including white space, letter spacing, and word spacing) except spacing (white space, letter spacing, and word spacing) at the beginning and end of a line. Atomic inlines, such as images and inline blocks, are not decorated. Margins, borders, and padding of the decorating box are always skipped, however the margins, border, and padding of descendant inline boxes are not.

Note that CSS 2.1 required skipping margins, borders, and padding always. In this level, by default only the margins, borders, and padding of the decorating box are skipped. In the future CSS2.1 may be updated to match this new default. Also, control over decorating leading/trailing spaces is expected in Level 4, and will be applied by default to the HTML ins and del elements.

UAs may interrupt underlines and overlines where the line would cross glyph ink and to some distance to either side of the glyph outline; this behavior is not controllable in this level, but will be further defined in Level 4. Line-throughs must remain continuous, however.

Skipping Glyph Ink

When the UA interrupts underlines or overlines at glyph boundaries, the shape of the line at that boundary should follow the shape of the glyph.

Note, this specification intentionally does not mandate a particular method for “following the shape” of the glyph so that UAs can take appropriate measures to handle aesthetic and performance considerations. For example, a UA could assume square line endings below a certain size threshold for performance reasons; or use trapezoidal endings to approximate curves, especially on thinner line decorations. In terms of aesthetic considerations, the UA might also consider what happens when the glyph boundary intersects only part of the line thickness or is slanted close to the horizontal—following the curve exactly could result in typographically-awkward wisps of underline. Whether to show the line within enclosed areas of a glyph is yet another consideration.

Hiding the portion of the underline within the bowl gives a cleaner look to the type, while the curved ends of the underline outside it suggest the continuity of the underline through the letter by hugging its outer contour.

Relatively positioning a descendant moves all text decorations applied to it along with the descendant’s text; it does not affect calculation of the decoration’s initial position on that line. The visibility property, text-shadow, filters, and other graphical transformations likewise also affect all text decorations applied to that box—including decorations propagated from an ancestor box—and do not affect the calculation of their initial positions or thicknesses. (In the case of line decorations drawn over an atomic inline or across the margins/borders/padding of a non-replaced inline box, they are analogously associated with the affected atomic inline / non-replaced inline box rather than with the decorating box.)

blockquote { text-decoration: underline; color: blue; }
em { display: block; }
cite { color: fuchsia; }

<blockquote>
 <p>
  <span>
   Help, help!
   <em> I am under a hat! </em>

   <cite> —GwieF </cite>
  </span>
 </p>
</blockquote>

Note: Line decorations are propagated through the box tree, not through inheritance, and thus have no effect on descendants when specified on an element with display: contents.

2.1. Text Decoration Lines: the text-decoration-line property

Specifies what line decorations, if any, are added to the element. Values have the following meanings:

none

Neither produces nor inhibits text decoration.

underline

Each line of text is underlined.

overline

Each line of text has a line over it (i.e. on the opposite side from an underline).

line-through

Each line of text has a line through the middle.

blink

The text blinks (alternates between visible and invisible). Conforming user agents may simply not blink the text. Note that not blinking the text is one technique to satisfy checkpoint 3.3 of WAI-UAAG. This value is deprecated in favor of Animations [CSS-ANIMATIONS-1].

Note: In vertical writing modes, text-underline-position can cause the underline and overline to switch sides. This allows the position of underlines to key off of language-specific preferences automatically.

2.2. Text Decoration Style: the text-decoration-style property

This property specifies the style of the line(s) drawn for text decoration specified on the element. Values have the same meaning as for the border-style properties [CSS-BACKGROUNDS-3]. wavy indicates a wavy line.

The style of text decorations must remain the same on all decorations originating from a given element, even if descendant boxes have different specified styles.

2.3. Text Decoration Color: the text-decoration-color property

This property specifies the color of text decoration (underlines overlines, and line-throughs) set on the element with text-decoration-line.

The color of text decorations must remain the same on all decorations originating from a given element, even if descendant boxes have different specified colors.

2.4. Text Decoration Shorthand: the text-decoration property

This property is a shorthand for setting text-decoration-line, text-decoration-color, and text-decoration-style in one declaration. Omitted values are set to their initial values.

Note: A text-decoration declaration that omits both the text-decoration-color and text-decoration-style values is backwards-compatible with CSS Levels 1 and 2.

:link {
  color: blue;
  text-decoration: underline;
  text-decoration: navy dotted underline; /* Ignored in CSS1/CSS2 UAs */
}

Note: The shorthand purposefully omits the text-underline-position property, which is a language/writing-system–dependent setting that keys off the content, so that it can cascade and inherit independently from the (uninherited) stylistic settings of the text-decoration shorthand.

2.5. Text Underline Position: the text-underline-position property

This property sets the position of an underline specified on the element. (It does not affect underlines specified by ancestor elements.) If left or right is specified alone, auto is also implied.

:root:lang(ja), [lang|=ja], :root:lang(ko), [lang|=ko] { text-underline-position: under right; }
:root:lang(zh), [lang|=zh] { text-underline-position: under left; }

Values have the following meanings:

auto

The user agent may use any algorithm to determine the underline’s position; however it must be placed at or under the alphabetic baseline.

Note: It is suggested that the default underline position be close to the alphabetic baseline, unless that would either cross subscripted (or otherwise lowered) text or draw over glyphs from Asian scripts such as Han or Tibetan for which an alphabetic underline is too high: in such cases, shifting the underline lower or aligning to the em box edge as described for under may be more appropriate.

A typical “alphabetic” underline is positioned just below the alphabetic baseline

under

The underline is positioned under the element’s text content. In this case the underline usually does not cross the descenders. (This is sometimes called “accounting” underline.) This value can be combined with left or right if a particular side is preferred in vertical typographic modes.

text-underline-position: under

Because text-underline-position inherits, and is not reset by the text-decoration shorthand, the following example switches the document to use under underlining, which can be more appropriate for writing systems with long, complicated descenders. It is also often useful for mathematical or chemical texts that use many subscripts.

:root { text-underline-position: under; }

Note: The under value does not guarantee that the underline will not conflict with glyphs, as some fonts have descenders or diacritics that extend below the font’s descent metrics.

left

In vertical typographic modes, the underline is aligned as for under, except it is always aligned to the left edge of the text. If this causes the underline to be drawn on the "over" side of the text, then an overline also switches sides and is drawn on the "under" side.

right

In vertical typographic modes, the underline is aligned as for under, except it is always aligned to the right edge of the text. If this causes the underline to be drawn on the "over" side of the text, then an overline also switches sides and is drawn on the "under" side.

left	right

In vertical typographic modes, the text-underline-position values left and right allow placing the underline on either side of the text. (In horizontal typographic modes, both values are treated as auto.)

The exact position and thickness of line decorations is UA-defined in this level. However, for underlines and overlines the UA must use a single thickness and position on each line for the decorations deriving from a single decorating box.

vs.

Correct and incorrect rendering of <u>A<sup>B</sup><big>C</big>D</u>

Note, since line decorations can span elements with varying font sizes and vertical alignments, the best position for a line decoration is not necessarily the ideal position dictated by the decorating box. For example, an overline positioned to a small font will effectively become a line-through if the element contains text in a significantly larger font-size. Even for underlines, if the text is not aligned to the alphabetic baseline (for example, in vertical typesetting styles, text is aligned by its central baseline by default [CSS-WRITING-MODES-4]) an underline will cut through descendant text of a larger font-size. UA consideration of descendant content will therefore result in better typography.

Due to the central baseline alignment of vertical text, a left-side underline on small vertical text will cut through the text of a child with a larger font size. The underline is not allowed to be broken, but adjusting its position further to the left properly accommodates all of the underlined text.

UAs must adjust line positions to match the shifted metrics of decorating boxes shifted with vertical-align values other than baseline [CSS2] or subscripted/superscripted via font-variant-position [CSS-FONTS-3], but must not adjust the line position or thickness in response to descendants of a decorating box that are so styled. This allows superscripts and subscripts to be properly decorated (underlined, struck through, etc.) but prevents them from distorting or breaking the positioning of such decorations on their ancestors.

Example of underline applied to superscripted text vs. underline applied to text containing a superscript

Some font formats (such as OpenType) can offer information about the appropriate position of a line decoration. The UA should use such information (such as the underline thickness, or appropriate alphabetic underline position) from the font wherever appropriate.

Note: Typically, OpenType font metrics give the position of an alphabetic underline; in some cases (especially in CJK fonts), it gives the position of a under left underline. (In this case, the font’s underline metrics typically touch the bottom edge of the em box). The UA may but is not required to correct for incorrect font metrics.

3. Emphasis Marks

East Asian documents traditionally use small symbols next to each glyph to emphasize a run of text. For example:

Accent emphasis (shown in blue for clarity) applied to Japanese text

The text-emphasis shorthand, and its text-emphasis-style and text-emphasis-color longhands, can be used to apply such marks to the text. The text-emphasis-position property, which inherits separately, allows setting the emphasis marks’ position with respect to the text.

3.1. Emphasis Mark Style: the text-emphasis-style property

This property applies emphasis marks to the element’s text. Values have the following meanings:

none

No emphasis marks.

filled

The shape is filled with solid color.

open

The shape is hollow.

dot

Display small circles as marks. The filled dot is U+2022 '•', and the open dot is U+25E6 '◦'.

circle

Display large circles as marks. The filled circle is U+25CF '●', and the open circle is U+25CB '○'.

double-circle

Display double circles as marks. The filled double-circle is U+25C9 '◉', and the open double-circle is U+25CE '◎'.

triangle

Display triangles as marks. The filled triangle is U+25B2 '▲', and the open triangle is U+25B3 '△'.

sesame

Display sesames as marks. The filled sesame is U+FE45 '﹅', and the open sesame is U+FE46 '﹆'.

<string>

Display the given string as marks. Authors should not specify more than one character in <string>. The UA may truncate or ignore strings consisting of more than one grapheme cluster.

If a shape keyword is specified but neither of filled nor open is specified, filled is assumed. If only filled or open is specified, the shape keyword computes to circle in horizontal typographic modes and sesame in vertical typographic modes.

The marks should be drawn using the element’s font settings with the addition of the ruby feature and the size scaled down 50%. However, since not all fonts have all these glyphs, and some fonts use inappropriate sizes for emphasis marks in these code points, the UA may opt to use a font known to be good for emphasis marks, or the marks may instead be synthesized by the UA. Marks must remain upright in vertical typographic modes: like CJK characters, they do not rotate to match the writing mode. The orientation of marks in horizontal typographic modes of vertical writing modes is undefined in this level (but may be defined in a future level if definitive use cases arise).

Note: One example of good fonts for emphasis marks is Adobe’s open source Kenten Generic OpenType Font, which is specially designed for the emphasis marks.

The marks are drawn once for each typographic character unit. However, emphasis marks are not drawn for:

• Word separators or other characters that belong to the Unicode separator classes (Z*). (But note that emphasis marks are drawn for a space that combines with any combining characters.)
• Punctuation--specifically, any characters that belong to the Unicode P* general category and do not NFKD normalize [UAX15] to any of the following symbols:

  #	U+0023	NUMBER SIGN
  %	U+0025	PERCENT SIGN
  ‰	U+2030	PER MILLE SIGN
  ‱	U+2031	PER TEN THOUSAND SIGN
  ٪	U+066A	ARABIC PERCENT SIGN
  ؉	U+0609	ARABIC-INDIC PER MILLE SIGN
  ؊	U+060A	ARABIC-INDIC PER TEN THOUSAND SIGN
  &	U+0026	AMPERSAND
  ⁊	U+204A	TIRONIAN SIGN ET
  @	U+0040	COMMERCIAL AT
  §	U+00A7	SECTION SIGN
  ¶	U+00B6	PILCROW SIGN
  ⁋	U+204B	REVERSED PILCROW SIGN
  ⁓	U+2053	SWUNG DASH
  〽️	U+303D	PART ALTERNATION MARK

• Characters belonging to the Unicode classes for control codes and unassigned characters (Cc, Cf, Cn).

Note: Control over which characters are marked will be added in Level 4. (The list of punctuation may also be further refined, particularly for non-CJK punctuation.)

3.2. Emphasis Mark Color: the text-emphasis-color property

This property specifies the foreground color of the emphasis marks.

Note: currentcolor keyword computes to itself and is resolved to the value of color after inheritance is performed. This means text-emphasis-color by default matches the text color even as color changes across elements.

3.3. Emphasis Mark Shorthand: the text-emphasis property

This property is a shorthand for setting text-emphasis-style and text-emphasis-color in one declaration. Omitted values are set to their initial values.

Note that text-emphasis-position is not reset in this shorthand. This is because typically the shape and color vary, but the position is consistent for a particular language throughout the document. Therefore the position should inherit independently.

3.4. Emphasis Mark Position: the text-emphasis-position property

This property describes where emphasis marks are drawn at. If [ right | left ] is omitted, it defaults to right. The values have following meanings:

over

Draw marks over the text in horizontal typographic modes.

under

Draw marks under the text in horizontal typographic modes.

right

Draw marks to the right of the text in vertical typographic modes.

left

Draw marks to the left of the text in vertical typographic modes.

Emphasis marks are drawn exactly as if each character was assigned the mark as its ruby annotation text with the ruby position given by text-emphasis-position and the ruby alignment as centered. Note that this position may be adjusted if it would conflict with underline or overline decorations.

The effect of emphasis marks on the line height is the same as for ruby text.

Note, the preferred position of emphasis marks depends on the language. In Japanese for example, the preferred position is over right. In Chinese, on the other hand, the preferred position is under right. The informative table below summarizes the preferred emphasis mark positions for Chinese and Japanese:

Preferred emphasis mark and ruby position

Language	Preferred position		Illustration
	Horizontal	Vertical
Japanese	over	right
Korean
Mongolian
Chinese	under	right

If emphasis marks are applied to characters for which ruby is drawn in the same position as the emphasis mark, the emphasis marks are placed outside the ruby. This includes auto-hidden and empty ruby annotations.

Emphasis marks applied to 4 characters, with ruby also on 2 of them

ruby { text-emphasis: none; }

em { text-emphasis: dot; } /* Set text-emphasis for <em> elements */
em rt { display: none; }   /* Hide ruby inside <em> elements */

4. Text Shadows: the text-shadow property

This property accepts a comma-separated list of shadow effects to be applied to the text of the element. Values are interpreted as for box-shadow [CSS-BACKGROUNDS-3]. (But note that spread values and the inset keyword are not allowed.) Each layer shadows the element’s text and all its text decorations (composited together).

The shadow effects are applied front-to-back: the first shadow is on top. The shadows may thus overlay each other, but they never overlay the text itself. The shadow must be painted at a stack level between the element’s border and/or background (if present) and the elements text and text decoration. UAs should avoid painting text shadows over text in adjacent elements belonging to the same stack level and stacking context. (This may mean that the exact stack level of the shadows depends on whether the element has a border or background: the exact stacking behavior of text shadows is thus UA-defined.) It is undefined whether a given shadow layer shadows each glyph or decoration independently or if the text and/or decorations are flattened and then shadowed.

Unlike box-shadow, text shadows are not clipped to the shadowed shape and may show through if the text is partially-transparent. Like box-shadow, text shadows do not influence layout, and do not trigger scrolling or increase the size of the scrollable overflow area.

Note: The painting order of shadows defined here is the opposite of that defined in the 1998 CSS2 Recommendation.

The text-shadow property applies to both the ::first-line and ::first-letter pseudo-elements.

5. Painting Text Decorations

5.1. Painting Order of Text Decorations

As in [CSS2], text decorations are drawn immediately over/under the text they decorate, in the following order (bottommost first):

• shadows (text-shadow)
• underlines (text-decoration)
• overlines (text-decoration)
• text
• emphasis marks (text-emphasis)
• line-through (text-decoration)

Where line decorations are drawn across box decorations or atomic inlines, they are drawn over non-positioned content and just below any positioned descendants (immediately below layer #8 in CSS2.1 Appendix E).

5.2. Overflow of Text Decorations

Text decorations that leak outside a box are considered ink overflow: they do not extend the scrollable overflow area. [css-overflow-3]

Appendix A: Acknowledgements

This specification would not have been possible without the help from: Ayman Aldahleh, Bert Bos, Tantek Çelik, Stephen Deach, John Daggett, Martin Dürst, Laurie Anna Edlund, Ben Errez, Yaniv Feinberg, Arye Gittelman, Ian Hickson, Martin Heijdra, Richard Ishida, Masayasu Ishikawa, Michael Jochimsen, Eric LeVine, Ambrose Li, Håkon Wium Lie, Chris Lilley, Ken Lunde, Nat McCully, Shinyu Murakami, Paul Nelson, Chris Pratley, Marcin Sawicki, Arnold Schrijver, Rahul Sonnad, Michel Suignard, Takao Suzuki, Frank Tang, Chris Thrasher, Etan Wexler, Chris Wilson, Masafumi Yabe and Steve Zilles.

Appendix B: Default UA Stylesheet

This appendix is informative, and is to help UA developers to implement default stylesheet, but UA developers are free to ignore or change.

/* typical styling of HTML */
blink {
  text-decoration-line: blink;
}
s, strike, del {
  text-decoration: line-through;
}
u, ins, :link, :visited {
  text-decoration: underline;
}
abbr[title], acronym[title] {
  text-decoration: dotted underline;
}

/* disable inheritance of text-emphasis marks to ruby text:
  emphasis marks should only apply to base text */
rt { text-emphasis: none; }

/* set language-appropriate default emphasis mark position */
:root:lang(zh), [lang|=zh] { text-emphasis-position: under right; }
[lang|=ja], [lang|=ko]     { text-emphasis-position: over right; }

/* set language-appropriate default underline position */
:root:lang(ja), [lang|=ja],
:root:lang(mn), [lang|=mn],
:root:lang(ko), [lang|=ko] { text-underline-position: right; }
:root:lang(zh), [lang|=zh] { text-underline-position: left;  }
/* auto is chosen (implied) above instead of under
   due to content-compatibility concerns */

If you find any issues, recommendations to add, or corrections, please send the information to [email protected] with [css-text-decor] in the subject line.

@keyframes blink {
  0% {
    visibility: hidden;
    animation-timing-function: step-end;
  }
  25%, 100% {
    visibility: visible;
  }
}
blink {
  animation: blink 1s infinite;
}

Appendix C: Changes

Changes since the August 2019 Candidate Recommendation

Changes include:

• Clarify that text-shadow animates as a shadow list like box-shadow. (Issue 4375)
• Be explicit about properties that apply to text. (Issue 5303)
• Minor editorial fixes.

Changes since the July 2018 Candidate Recommendation

Changes include:

• Clarified that text decoration overflow is ink overflow. (Issue 3272)
• Fixed inconsistencies in sample text-underline-position rules. (Issue 3441)
• Cleaned up “Computed value” lines to match new conventions.

A Disposition of Comments is available.

Changes since the August 2013 Candidate Recommendation

Significant changes include:

• Deferred text-decoration-skip to Level 4 to allow for major changes. Defined behavioral defaults in prose. (Issue 1, Issue 22, Issue 26)
• Specified that line-throughs are unaffected by ink-skipping feature. (Issue 24)
• Recommended that when ink is skipped, line endings conform to the glyph shape. (Issue 30)
• Updated writing-mode–sensitive conditions to depend on typographic mode, to account for addition of sideways-lr and sideways-rl values to writing-mode property. Marked orientation of emphasis marks under sideways-lr and sideways-rl undefined. (Issue 10, Issue 20)
• Made [ right | left ] option of text-emphasis-position optional, defaulting to right. (Issue 17)
• Made text-underline-position imply auto instead of under when only left or right is specified. (Issue 18)
• Changed text decoration to skip leading and trailing spaces. (Issue 6)
• Noted that the positions of ruby annotations may be adjusted to avoid conflicts with text decorations. (Issue 21)
• Changed initial value of text-shadow to be currentColor. (Issue 28)
• Fixed error in “Computed value” line for text-shadow. (Issue 7)
• Fixed canonical order of text-shadow values to match implementations. (Issue 35)
• Defined positioning of emphasis marks with respect to auto-hidden and empty ruby annotations. (Issue 9)

  If emphasis marks are applied to characters for which ruby is drawn in the same position as the emphasis mark, the emphasis marks are placed outside the ruby. This includes auto-hidden and empty ruby annotations.

• Made text-emphasis skip punctuation by default. (Issue 16)
• Added rule to apply ruby to emphasis marks' font. (Issue 13)
• Various corrections and improvements to the default UA rules for text-emphasis-position and text-underline-position. (Issue 11, Issue 12, Issue 18, Issue 19, Issue 36)